1) Field of the Invention
The subject of the present invention is a silica-enriched activated mazzite obtained by a crystallization gel containing zeolite crystals. The present invention also relates to its process of preparation and to its application as reaction catalyst for the conversion of hydrocarbons, in particular by isomerization, or as molecular sieves.
2) Background Art
Obtained for the first time by a synthetic route in 1966 under the name of zeolite omega by Flanigen and Kellberg (U.S. Pat. No. 4,241,036), mazzite was identified in 1972 in basaltic rocks from Mont Semiol, near Montbrison, Loire, France. Its crystalline structure was resolved by Galli (Cryst. Structure Comm., 3, 339, 1974) and Rinaldi et al. (Acta Cryst., B31, 1603, 1974). From its structure, of hexagonal symmetry, it belongs to the category of highly acidic zeolites with broad pores and unidirectional porosity, which makes it particularly advantageous for applications in catalysis, in particular for the conversion of hydrocarbons.
The omega claimed by Flanigen (U.S. Pat. No. 4,241,036) is characterized by an X-ray spectrum common to all the types of mazzites prepared subsequently, such as ZSM-4, LZ 202 or MZ-34. However, these mazzites, although identical in structure, are distinguished from one another by their specific synthetic route and different physical characteristics, such as the Si/Al ratio, their specific surface and their porosity.
In order to obtain a mazzite, so-called crystallization gels, containing a trivalent aluminium source, a silicon source, at least one alkali metal or alkaline earth metal cation in the hydroxide form, water and optionally an organic structuring agent, can be formed. However, from these constituents, by varying the stoichiometry of the gel or the conditions of the subsequent hydrothermal treatment necessary for the crystallization, it is possible to obtain zeolites which are very different from mazzite, such as offretite (OFF) or zeolite L (LTL) (S. Ernst and J. Weitkamp, Catalysis Today, 19, 1994, 27-60).
In order to improve the physical characteristics of these mazzites, silica gels, colloidal silicas, precipitated silicas, silicates or hydrolyzable silicic esters have been introduced into the synthesis or crystallization gel, as silicon source, and aluminates, alumina hydroxides, alumina in the pure or commercial form or amorphous aluminosilicates have been introduced into the synthesis or crystallization gel as trivalent aluminium sources. These aluminium sources were subsequently replaced by natural or synthetic crystalline aluminosilicates, mainly natural clays. Such a substitution has made it possible to obtain homogeneous growth of the crystals, due to the slow and even dissolution of these aluminosilicates in the synthesis medium (Dwyer, U.S. Pat. No. 4,091,007; Fajula, U.S. Pat. No. 4,891,200).
Other methods have been developed for synthesizing a novel mazzite from seeds of mazzite or of another zeolite in the presence or in the absence of organic structuring agent containing alkylammonium ions (Cannan, U.S. Pat. No. 4,840,779, Di Renzo, FR 2,651,221 and FR 2,698,862).
Thus, by varying the crystallization temperature to between 90.degree. C. and 150.degree. C. and the content of sodium cations and of organic structuring agent, such as tetramethylammonium, choline or p-dioxane, in the crystallization gels, it has been possible to obtain the various known types of mazzite called omega, ZSM-4, LZ 202 or MZ-34 (cf W. M. Meier and D. H. Olson, "Atlas of Zeolite Structure Types", Third Revised Edition, Butterworth, London 1992).
However, whatever the method of synthesis employed above, it has been impossible to synthesize a mazzite exhibiting an Si/Al ratio in the precursors or alternatively crude synthetic products greater than 5, these ratios generally being between 2.5 and 5, which corresponds to a molar aluminium concentration varying between 0.166 and 0.285. Now, the combination of a high aluminium content and of a unidirectional porosity does not favour the application of such an unactivated mazzite in catalysis, because the desired optimum Si/Al ratio is frequently greater than 10 for such an application, which can only be obtained by dealumination during an activation phase.
Activation phase is usually understood to mean the combined individual stages carried out after the synthesis of a zeolite and which are targeted at rendering it active in catalysis and in adsorption. These individual stages, generally preceded by a stage of calcination of the crude synthetic zeolite precursor, comprise an ion exchange, then a hydrothermal treatment and an acidic washing.
These treatments are known to induce profound textural and structural modifications in zeolites, and mazzites in particular, which affect their porosity and their acidity. Thus, the calcination stage, intended to decompose the organic structuring agent occluded within the pores of the mazzite precursor, causes partial degradation of the structure and the formation of amorphous residues which remain trapped in the pores and which obstruct the channels of the mazzite. The lower the Si/Al ratio in the precursor, the greater the loss in crystallinity and the greater the amount of amorphous residues. This limits the catalytic efficiency of the mazzite and the effectiveness of the subsequent activation treatments. In addition, the hydrothermal treatment and acidic washing stages result in the formation of non-bridging bonds, creating silanol defects which decrease the strength and the number of acid centres, reducing the long-distance order in the zeolite lattice.
The aim of the present invention is to avoid the problems encountered by mazzites with an excessively low Si/Al ratio, which, after activation, contain amorphous residues and lattice defects which reduce the catalytic performances, and the present invention is targeted at the production of a silica-enriched activated mazzite with improved properties and with a limited number of silanol defects in the lattice which exhibits a larger number of available acid sites and in particular an increased acid strength.
The subject of the present invention is a silica-enriched activated mazzite with a chemical formula, in the anhydrous state, expressed as molar ratio, of EQU aM.sub.2/n O.Al.sub.2 O.sub.3.bSiO.sub.2
with a varying from 0 to 0.5, M denoting an alkaline cation of valency n and b being greater than 7, characterized in that it exhibits an acid strength, expressed as amount of heat of adsorption of ammonia, greater than 190 kJ/mol and a pore volume, measured by adsorption of cyclohexane, greater than 0.09 ml/g.
According to the present invention, the acid strength of the activated mazzite corresponds to the amount of initial heat of adsorption of ammonia; it is measured by microcalorimetry. The measurement consists in adsorbing gaseous ammonia on the activated mazzite at 150.degree. C. and in measuring the amount of heat released. The activated mazzite according to the invention exhibits a particularly significant acid strength because, until now, whatever the method of synthesis known and employed by a person skilled in the art, it has never been possible to obtain as high a value of the amount of heat of adsorption of ammonia on an activated mazzite.
These measurements of acidity of the mazzite from the amounts of heat of absorption of ammonia are fully described in the article entitled "A multitechnique characterization of the acidity of dealuminated mazzite" by D. McQueen, B. H. Chiche, F. Fajula, A. Auroux, C. Guimon, F. Fitoussi and Ph. Schulz, J. Catal., 1996, 161.
In order to obtain such an acid strength, the activated mazzite according to the invention is obtained from a zeolite precursor itself synthesized from a crystallization gel containing faujasite X, with an Si/Al ratio of less than 1.5, the said precursor subsequently being subjected to an activation treatment.
The Applicant Company had already synthesized mazzite from grains of faujasite Y with an Si/Al ratio of greater than 1.5, as is described in Patent FR 2,698,862. However, in contrast to what it had recommended, the molar composition of the crystallization gel is completely different; in fact, the molar ratios vary
from 5 to 15 for SiO.sub.2 /Al.sub.2 O.sub.3 PA1 from 1 to 2 for SiO.sub.2 /TMA.sub.2 O PA1 from 0.03 to 0.25 for TMA.sub.2 O/Na.sub.2 O PA1 and from 30 to 150 for H.sub.2 O/Na.sub.2 O PA1 from 5 to 15 for SiO.sub.2 /Al.sub.2 O.sub.3 PA1 from 1 to 2 for SiO.sub.2 /TMA.sub.2 O PA1 from 0.03 to 0.25 for TMA.sub.2 O/Na.sub.2 O PA1 and from 30 to 150 for H.sub.2 O/Na.sub.2 O
with TMA.sub.2 O, the organic structuring agent, chosen from tetraalkylammonium ions, each alkyl group comprising from 1 to 4 carbons and preferably denoting the tetramethylammonium ion. The sodium is introduced partly by the addition of sodium hydroxide used to adjust the alkalinity of the gel and the tetravalent silicon and trivalent aluminium sources originate partly from the group composed of silicates, solid or colloidal silicas, gels and xerogels, hydrolyzable silicic esters and diatomites and, on the other hand, from faujasite X. In the gel, it is possible, however, to have therein a mixture of alkaline ions, it being possible for the latter in particular to be introduced by faujasite X.
The use of commercially available faujasite X has made it possible to reduce both the cost of the starting materials and the cost of manufacture of the zeolite of mazzite type. This is because it is no longer necessary to form grains with a specific configuration which are delimited solely by rounded surfaces, the synthesis of which is expensive, in particular in time (10 to 12 days of crystallization at 50.degree. C., with or without stirring), as was described in Patent FR 2,698,862.